Underbalanced drilling is defined as drilling operations where the drilling fluid pressure in the borehole is less than the pore pressure in the formation rock in the open-hole section. The light fluids used in underbalanced drilling are usually air, gas, foam and aerated water. Unaerated oil, water and weighed mud are used for underbalanced drilling in areas where formation pore pressure gradients are higher than the hydrostatic pressure gradient of water. Underbalanced drilling technology is an integrated technology, resulting from merging traditional air and gas-drilling techniques with aerated & foam drilling method. These drilling avoid overbalanced pressure to the rock below the drill bit. Underbalanced drilling is an effective means of minimizing lost circulation problems in drilling naturally fractured petroleum reservoirs and pressure-depleted reservoirs.